1. Field of the Invention
This invention relates to solenoid control, especially in the context of solenoid-controlled fuel injection systems in vehicle engines.
2. Background Art
In order to minimize the exhaust of particles and nitrous oxide (NOx), as well as to achieve the highest possible efficiency in a diesel engine, the crank angle position at which fuel-injection into a cylinder of a vehicle engine is initiated is critical. Because such fuel injection is typically controlled by a solenoid valve, it is not enough to ensure that the control signal occurs at the correct position; rather one must also know when the valve itself has reached its fully opened position. One known method for determining this involves measuring the current in the driving stage of the solenoid and therefrom detecting the change in inductance that arises when the valve cone is seated.
This method is usually referred to as BIP-detection, where BIP stands for “Beginning of Injection Pulse.” FIG. 1 is a diagram of current and voltage as functions of time as used in the conventional BIP technique. In principle, the solenoid is controlled by applying a voltage pulse U until the current in the solenoid winding reaches a predetermined level known as the “pull-in” current, which is the current level that must be achieved in the circuit in order to be able to move the solenoid armature.
Thereafter, the control voltage U is pulsed so that the winding current remains approximately at this level until the valve is fully opened. Once the valve is fully open, however, a significantly lower current—the so-called “hold” current—is needed in order to keep the valve open. This hold current is also maintained by pulsing the control voltage U. The hold current is maintained until it is once again time to close the valve, which is determined by the amount of fuel that is to be injected.
Detecting the BIP signal at the same time as the pull-in current is being regulated is very difficult because the BIP signal is typically obscured by the noise that arises when using such pure current regulation. The application of the pull-in current is therefore usually turned off immediately before the time when the BIP signal is expected to arise, which can be estimated using known methods. The BIP signal (which appears as a “bump” in the current curve) then occurs in the period during which the current discharges through a freewheel diode D connected to the solenoid winding. This period of current “decay” is known as the BIP “window.” The minimum width of the BIP window needed for reliable detection of the BIP using standard equipment is typically about 600 μs.
“Freewheeling” refers to the remaining current that circulates within the solenoid circuit after the applied voltage has been shut off. If there were no resistive losses in this circuit, the freewheeling could theoretically continue forever. Components such as a freewheeling diode D and at least one resistive shunt are usually included in the solenoid circuitry, however. It has, moreover, also been shown that the time it takes for the solenoid current to decrease from the pull-in level to the hold level can vary greatly in practice, primarily because of resistances in the network of conductors (such as cables) and connectors used to connect the various components in the circuitry involved in operating the solenoid. These conductor resistances vary not only from application to application, but even among different valves in the same engine. The time for BIP detection may therefore be too short, such that it may become impossible to detect the occurrence of the BIP with certainty—the BIP pulse may fall outside the BIP window and disappear in the noise created by the current regulation.
The main components of a typical prior art circuit that implements current-only control are shown in FIG. 3. The injection solenoid S (represented in the figures as its inductive winding) is usually connected to a system power supply V via a resistive shunt Rs, in parallel with a freewheel diode D. A conventional circuit 100 is included to measure current through the solenoid, the result of which is applied to a differencing component (shown as an operational amplifier 202) in a current-regulating circuit 200.
Usually, this circuit 200 will have two inputs, namely, one to set the desired current level and another to turn the current on and off completely. The difference between measured current and desired current is then “added” into the circuit using a power transistor Q1. The On/Off signal is similarly applied via a corresponding transistor Q2, which acts essentially as a switch.
The source of the input signals for current level and current ON/OFF will typically be a supervisory processor that calculates desired values and times and generates the input signals in digital form, which are the converted into analog form using a conventional digital-to-analog converter.
The reason that the voltage U to the solenoid circuit is pulsed ON/OFF in the prior art, instead of being controlled over a continuous range is that the power that develops in the control electronics becomes too high. The problem to be solved is therefore how to ensure a sufficiently large BIP window, thereby allowing reliable BIP detection, without too much power being developed in the circuitry. One known attempted solution to this problem is to include additional circuitry that adds voltage directly to the free-wheeling circuit. The difficulties and complications associated with this solution are well known.